Motor vehicles increasingly have systems for monitoring and/or measuring parameters comprising sensors mounted on said vehicle.
By way of example regarding such systems, mention can be made of systems for monitoring the pressure in the tires comprising sensors mounted on each of the wheels of the vehicle, dedicated to the measurement of parameters, such as the pressure and/or temperature of the tires fitted to those wheels, and intended to inform the driver of any abnormal variation of the measured parameter.
These monitoring systems conventionally comprise:                mounted on each of the wheels of the vehicle, an electronic housing integrating the measuring sensors, a microprocessor and a radio transmitter,        and, mounted on the vehicle, a central unit for receiving the signals transmitted by the electronic housings, provided with a computer integrating a radio receiver connected to an antenna.        
In the prior art, JP 2007 237781 A, there is known the use of four RF (radio-frequency) radio receivers, instead of a single one integrated in the central unit, disposed close to each housing and connected electronically to the central unit. These receivers receive the signals transmitted by the housings and retransmit them to the central unit. Such a system has the advantage of not being very sensitive to parasitic noise, but they remain relatively expensive because of the use of four receivers instead of a single one situated in the central unit.
One of the techniques that can be envisaged for the purpose of transmitting signals from electronic housings to the central unit consists in the spread spectrum technique which makes it possible to transmit the signals on a bandwidth several times bigger than the bandwidth of a narrow band signal.
For this purpose, according to this spread spectrum technique, each transmitted signal is spread on the basis of a code which is independent of the transmitted data, a code which is also used, on reception, for “unspreading” (decoding) the signals and allowing the retrieval of the useful data.
Such a spread spectrum technique notably proves to be very insensitive to parasitic noise and very efficient in the presence of multiple paths, these properties advantageously resulting in:                improving the performance of the procedures for location of the electronic housings and therefore reducing the time required by these procedures,        improving the radio-frequency performance during the transmission of signals representing the operational parameters of the wheels.        
However, if the communications between each housing and the central unit are all carried out according to the spread spectrum technique using a single identical spread/unspread code and if there is collision between the different messages (which is relatively frequent since there is no synchronization between the times of transmission of the various messages from the wheel units), it then becomes impossible to decode each message sent by the electronic housings and to allocate a predetermined origin to each message (location of each wheel). The impossibility of decoding the received signals makes this type of communication with a single spread code impossible to use.
In order to prevent such collisions, there is known in the prior art, DE 10 2006 009 899, a receiver of radio-frequency waves whose bandwidth is adjustable according to the types of signal it receives. For example, a first type of signal comes from the “hands free” vehicle door opening system, that is to say a system having no key and using a badge transmitting a radio-frequency signal and a second type of signal comes from the tire pressure monitoring system, equipped with the previously described housings. Each of these systems sends RF signals to this receiver by using different bandwidths. In order to receive these signals, the receiver modulates its receiving bandwidth on the basis of vehicle data, which apply to the system which must be activated. Thus, if the vehicle is stationary, the receiver modulates its bandwidth to that of the “hands free” door opening system in order to receive the corresponding signal and, similarly, if the vehicle is moving, the receiver modulates its bandwidth to that of the system for monitoring the pressure in the tires in order to receive the signal from this system. In this way collisions of RF waves between the different systems of the vehicle are avoided. However, such a receiver is not applicable to the tire monitoring system because it does not differentiate between the different sources in the context of one and the same function and consequently it cannot distinguish the signals sent by each one of the housings, because these send their signals when the vehicle is in one and the same condition, that is to say when moving.
On the contrary, if a spread spectrum technique is used with a spread/unspread code that is different for each electronic housing, then it is initially necessary for the central unit mounted on the vehicle to learn all spread/unspread codes of the housings of the vehicle. In this case, the procedure is longer and more complex.
This problem of learning spread/unspread codes could be avoided by using a conventional transmission procedure (RF, LF for example) for transmitting the various spread/unspread codes of each of the housings, and then a spread spectrum procedure for the subsequent transmissions (in order to benefit from the advantages of this type of transmission). However, this would necessitate equipping all vehicles with two different transmission means (one with spread spectrum and one without). The cost of the resultant system would thereby become prohibitive.